Prior art laser scanner systems frequently use mechanical scanners, such as polygon prisms, to scan an object. The production costs associated with polygon prisms are relatively high because of the angular precision needed in the reflecting surfaces. Alternative scanning techniques have therefor been sought in the prior art.
The utilization of rotating holographic gratings for image scanning was first disclosed by Cindrich in an article published in Applied Optics, Volume 6, page 1531 (1967). Subsequent thereto, McMahon et al (Applied Optics, Volume 8, page 399 (1969)) disclosed a two-dimensional raster scanning system using a set of rotating holograms. By using holograms, the mechanical system in their scanner was significally simplified. Also, since there was no restriction on the shape of the surface for recording holograms, the geometry of the scanning surface could be selected to achieve a high-speed and mechanically stable system. In general, the mechanical design of the scanner can be separated from its optical design with holographic techniques. Other advantages of holographic scanners include:
(1) accuracy (reliance only on positional accuracy) and economy in fabricating holographic gratings, PA0 (2) two or three-dimensional raster scanning, and PA0 (3) multi-wavelength raster scanning for color displays.
Prior art holographic scanners are shown, for example, in U.S. Pat. Nos. 3,614,193, 3,721,486 and 3,721,487.
The gratings in the prior art scanners have basically one spatial frequency. Scanning is done by changing the angular orientation of the grating. However, a light beam can also be deflected by varying the spatial frequency of the gratings in a manner similar to acoustic beam deflectors which provides a less complex system of scanning. Although the scanner disclosed in U.S. Pat. No. 3,721,486 achieves a variable spatial frequency grating by combining two gratings of a single spatial frequency, the requirement of an additional grating is an obvious limitation to its utilization. A space variant frequency grating can be constructed so that the varying deflection of the light beam is produced by moving different parts of the grating across the light beam as set forth by Wollenmann et al in the Journal of the Optical society of America, Volume 64, page 562 (1974). The space-variant frequency grating can be replaced by a glass wedge of variable slopes as described in U.S. Pat. Nos. 2,976,362 and 3,818,132, although the flexibility of the holographic grating has distinct advantages in laser scanning system. A prior art space-variant frequency grating which may be utilized for laser beam scanning is a Fresnel zone plate, the spatial frequency thereof varying linearly with its radius. In this system (Wollenmann et al), the grating is recorded by the interferometric techniques on a transparent drum coated with photoresist.